A transformer on no-load is switched on to a source of voltage. It will draw a current
Right Answer is:
Which is several times the steady-state magnetizing current depending upon the initial state of the residual flux in the transformer core.
A transformer on no-load is switched on to a source of voltage. It will draw a current which is several times the steady-state magnetizing current depending upon the initial state of the residual flux in the transformer core.
- Whenever a transformer is on no-load,i.e the secondary winding has no burden(load) connected to it.
- The transformer must actually withdraw zero current from the primary side.
- But practically even on no-load, a small amount of current is drawn from the primary side, to set up the required magnetic flux in the magnetic core.
- At no load, the transformer draws some current (Im) in order to establish the flux in the transformer core which lags the applied voltage by 90° (since flux is required to create a magnetic pool in order to transfer energy from one port to another) called as magnetizing current.
- This magnetizing current (no-load current) is about 3-5% of the full load current and it accounts for the losses in a transformer.
- When the transformer is in no-load condition, the primary carries a small no-load current while the secondary current is zero.
- However, difficulties arise when an unloaded or lightly loaded transformer is switched on to the supply by closing the breaker at the primary side.
- There may be a transient on the primary side immediately after the switching.
- The core flux may rise to a very high value (almost equal to twice the normal value) depending on the switching instant, for a few cycles afterward.
- This high core flux density drives the core to saturation and the primary winding draws a very large (may be even 7-10 times the rated current) magnetizing current depending upon the initial state of the residual flux in the transformer core.
- In some transformers, this may be as high as 50 to 100 times the normal magnetizing current.
- This is the inrush current which decays with time to the small no-load current.
- It is not an abnormal situation as the transformer can withstand such a large current for a few cycles without damage.